Individuals with borderline personality disorder (BPD) are the quintessential multi-problem patients, often presenting to treatment with numerous dysfunctional behaviors and comorbid diagnoses. Dialectical Behavior Therapy (DBT) is a comprehensive, cognitive-behavioral treatment for BPD that has been shown effective in reducing the primary problems it is designed to treat;namely, the frequency and severity of self-injurious and suicidal behavior, maintenance in treatment, and severe problems in living. However, the DBT treatment manual does not currently include a protocol specifying when or how to treat posttraumatic stress disorder (PTSD), a comorbid diagnosis that is prevalent in BPD patients and may maintain or exacerbate BPD criterion behaviors. Similarly, many of the existing treatment outcome studies for PTSD have excluded suicidal, substance abusing, and multiply diagnosed patients, thereby making it difficult to determine the generalizability of these approaches to individuals with BPD. The research proposed here is focused on the development of a protocol based on Prolonged Exposure therapy to treat PTSD in BPD patients that can be integrated into standard DBT, as well as the initial evaluation of this protocol's feasibility, acceptability, and efficacy. The treatment development and pilot testing process will occur in two phases, including measure development and standardization of the treatment protocol via clinical pre-testing (Phase 1);and pilot and feasibility testing of the intervention via a randomized controlled trial (RCT) comparing standard DBT + PTSD Protocol to standard DBT Only (Phase 2). Information gathered during the pilot RCT will be used to inform the design and conduct of a subsequent full-scale RCT. This research has the potential to significantly expand and improve upon the most empirically supported treatment currently available for BPD, while also demonstrating that exposure treatments for PTSD can be implemented safely and effectively in a BPD population. Borderline Personality Disorder (BPD) is a serious, often lethal, and expensive public health problem. It is critical that researchers continue to expand and improve upon existing efficacious treatments for BPD, and this application addresses this by developing a protocol for treating PTSD that can be added to the Dialectical Behavior Therapy manual.